Stress compensated transducer

ABSTRACT

A transducer having compensation for a deflection due to an applied stress. The transducer a support ring (32) having a proof mass (34) cantilevered on a pair of flexures (38) between the magnets (26, 28) of a stator in which the transducer is mounted. Deflection of the support ring due to an imbalanced applied force is compensated by either moving the pads (30) used to mount the support ring, moving the centroid of capacitance (128) of the proof mass, or by modifying the support ring to provide a pair of moment arms (156), each approach insuring that an axis of deflection (102, 130) of the support ring is coaligned with the centroid of capacitance, thereby minimizing a bias error in the transducer output.

This is a continuation application of the prior application Ser. No.07/212,785, filed June 29, 1988 now U.S. Pat. No. 4,932,258. The benefitof the filing dates of which are hereby claimed under 35 U.S.C. §120.

TECHNICAL FIELD

The present invention generally pertains to a transducer for detectingacceleration, and more particularly, to a transducer wherein a proofmass is mounted in cantilever fashion to a supporting structure that issubject to an imbalanced applied force.

BACKGROUND INFORMATION

In certain transducers of the prior art, one side of a supportingannular ring is clamped in cantilever fashion between two opposedstators. A pair of flexures extend inwardly from the opposite side ofthe ring to support a disk-like proof mass. The proof mass includes atorque coil mounted on each face, which upon displacement of the proofmass, operates to restore the mass to a centered position relative tothe stators. Surrounding the coil is a plated pick-off capacitance area.Electrical paths on the flexures connect the torque coil and pick-offcapacitance area to leads on the support. A representative example ofsuch a transducer is described in greater detail in U.S. Pat. No.4,250,757, assigned to the same assignee as the present invention.

A problem related to such transducers arises when a force is applied tothe supporting ring in a direction perpendicular to the plane of thering, causing the ring to deflect. The linear and angular deflection ofthe supporting ring is translated through the flexures to the proofmass, causing a centroid of capacitance, i.e. the effective center ofthe pick-off capacitance area for small displacements, to be displacedfrom its normal position wherein it is approximately centered betweenthe top and bottom stators. The torque coil reacts to the displacementof the centroid of capacitance by restoring the proof mass to its priorposition. However, because there has been a repositioning of the proofmass resulting in bending of the flexures, a continuous restoring torqueis required to balance the moment applied by the flexures. Consequently,the output signal from the transducer includes a bias shift component.

External imbalanced forces applied to the proof mass supportingstructure can result from a variety of causes. For example: (a) the goldfly wires that connect to the support may exert a residual force whichrelaxes over time due to the creep characteristics of gold; (b) anelastic damping material applied to the support may produce animbalanced force on the structure, due to thermal variations in theenvironment; (c) static charge buildup can produce either an attractiveor repulsive force between the support and an adjacent surface; and, (d)preload variations and thermally variable distortion may result shouldthe cantilevered portion of the support contact an adjacent part of thestator through a contaminating particle or due to assembly error.

A dynamic source of force imbalance applied to the proof mass supportmay result from loading the support with a "g" force (force ofacceleration). In this instance, the bias shift is a linear function ofthe acceleration, and thus appears as a shift in the transducer scalefactor. Such an apparent shift in the scale factor occurring over timecan create a significant problem when the transducer is exposed tovibration at a frequency near the resonant frequency of the support. Theoverall effect of such a dynamically induced loading on the supportmanifests itself as a vibration rectification error at certain frequencyranges.

Whether resulting from static or dynamically induced imbalanced loading,deflection of the supporting element can cause an undesirable bias shiftor error signal in the output of transducers of the prior art typedescribed above. The present invention seeks to compensate fordeflection of the support due to such force, whatever its cause, andthereby to minimize bias shift and dynamic signal error in thetransducer output that might otherwise result.

SUMMARY OF THE INVENTION

The present invention applies to a transducer of the type describedabove, wherein a movable proof mass has a plated pick-off capacitancearea on its surface. Associated with the pick-off capacitance area is acentroid of capacitance. The proof mass is attached by a compliantcantilever arm to a side of the support and is thus movable in agenerally transverse direction relative to a plane aligned with asurface of the support. The support is mounted in cantilever fashionwithin a stator assembly.

The transducer further comprises means for detecting a change in motionof the transducer along the transverse direction by sensing adisplacement of the pick-off capacitance area, means for producing arestoring signal to eliminate the displacement, and producing an outputsignal that is a function of the restoring signal and thus indicative ofthe change in motion.

An imbalanced force applied against the support, having a component inthe transverse direction, is compensated in several alternative waysaccording to the present invention. The basic premise of the inventionin providing such compensation is that the centroid of capacitance mustbe aligned with a deflection axis about which the support deflects underthe applied imbalanced force, so that the centroid of capacitance doesnot deflect because of that force. One approach to achieving thiscondition provides for changing the disposition of a plurality of padsdisposed between the support and the stator assembly within whichsupport is mounted. Instead of being spread widely apart, as in theprior art design, the pads are grouped closely together in a relativelyshort segment of the support, opposite the side at which the cantileverarm connects the pick-off capacitance to the support. Closely groupingthe mounting pads in this fashion shifts the axis about which thesupport deflects under the applied imbalanced force so that it isaligned with the centroid of capacitance.

In the instance where the imbalanced force is applied to the support ata point closer to one edge of the cantilever arm than to the other, thecompensation comprises provision of a larger capacitance area on a sideof the proof mass that is farther from that point than on a side that isnearer, thereby shifting the centroid of capacitance into alignment withthe deflection axis.

In a further alternative, the support includes first and second integralslots defining two moment arms extending generally toward a side of thesupport to which the imbalanced force is applied. Mounting pads aredisposed on the ends of the moment arms, between the stator assembly andthe support. The moment arms serve to shift the axis about which thesupport is deflected, so that it is aligned with the centroid ofcapacitance, and thus compensate for the imbalanced force applied to thesupport.

The invention further comprises a method for accomplishing thecompensation of an imbalanced force applied to the support, as describedabove.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a transducer to which the presentinvention is applied.

FIG. 2 shows in plan view, the proof mass and supporting structure of aprior art transducer.

FIG. 3 illustrates in an elevational view a displacement of the centroidof capacitance resulting from an applied force causing deflection(exaggerated) of the supporting ring of the prior art transducer shownin FIG. 2.

FIG. 4 shows in plan view, a first embodiment of the proof mass andsupporting ring structure comprising the present invention.

FIG. 5 shows in elevational view, the first embodiment shown in FIG. 4,following application of an imbalanced force to the supporting ring.

FIG. 6 is a plan view illustrating the proof mass and supporting ringstructure for a second embodiment of the present invention, wherein adeflecting force is applied adjacent an edge of a compliant flexure.

FIG. 7 shows an elevational view of the second embodiment of FIG. 6,illustrating the deflection (exaggerated) in the supporting structureresulting from the applied imbalanced force.

FIG. 8 shows in plan view, a third embodiment of the present invention.

FIG. 9 illustrates in elevational view, the result of an applied forcecausing deflection (exaggerated) of the supporting structure of thethird embodiment shown in FIG. 8.

FIG. 10 is a geometric representation of a cantilevered support arm andcantilevered proof mass.

FIG. 11 is an elevational view of the representation shown in FIG. 10,following application of a deflecting force.

FIG. 12 shows a fourth embodiment of the present invention, related tothe third embodiment, in a plan view.

FIG. 13 shows the fourth embodiment in elevational view.

FIG. 14 shows a fifth embodiment of the present invention in plan view.

FIG. 15 shows the fifth embodiment in elevational view.

Disclosure of the Preferred Embodiments

With reference to FIG. 1, a transducer 20 is shown, which is useful forproducing a signal indicative of acceleration directed along a preferredaxis. Transducer 20 includes first and second stators 22 and 24,respectively, which comprise corresponding first and second magnets 26and 28, disposed in opposed axial alignment. Stators 22 and 24compressively abut pads 30 disposed on opposite surfaces of a supportring 32. Support ring 32 is thus mounted between stators 22 and 24 alongone side of its circumference.

A disk-like proof mass 34 is mounted in cantilever fashion inside theinner circumference of support ring 32 by a cantilever arm comprisingflexures 38 which extend from the inner circumference of support ring 32toward the side at which are disposed mounting pads 30. Proof mass 34has a generally planar surface from which a cylindrical torque coil 36extends on each side to partially enclose magnets 26 and 28. Oppositesurfaces of proof mass 34 (outside the circumference of torque coil 36)are coated with a metallic layer comprising a pick-off capacitance area40. Associated with the pick-off capacitance area 40 is a centroid ofcapacitance 42, defined as the capacitive center of the pick-offcapacitance area.

A prior art support ring and proof mass assembly is shown in FIG. 2,wherein the centroid of capacitance including the stray capacitance ofadjacent conducting surfaces is generally aligned with a line extendingbetween the most widely disposed support pads 30. The pick-offcapacitance area 40 is electrically connected through a conductive path52 comprising a metallic layer that extends from pick-off capacitancearea 40 across flexures 38 onto support ring 32. A gold fly wire 50contacts an end of the conductive path 52 and provides a conductive pathto the stator assembly.

The prior art support ring 32 is mounted in widely spaced-apart pads 30(i.e., the end pads 30 being almost aligned with a diameter of thesupport ring, as shown in FIG. 2), and may be subject to an imbalancedforce applied as indicated by arrow 44 in FIG. 3, having a componentdirected parallel to the preferred axis. The various sources and causesof such a deflecting force were discussed hereinabove, and need not berepeated. As shown in an exaggerated fashion in FIG. 3, force 44 causessupport ring 32 to deflect downwardly. The downward deflection ofsupport ring 32 results in an upwardly directed vertical displacement 48of the centroid of capacitance 42. In response to displacement 48 of thecentroid of capacitance 42 from its "normal" or original position, i.e.,centered between first and second stators 22 and 24, torque coils 36produce a magnetic restoring force relative to first and second magnets26 and 28 that causes a bending moment about flexures 38, restoring thecentroid of capacitance 42 to its original position. As a result of therestoring action produced by torque coils 36, the output signal from theprior art transducer shown in FIG. 3 includes a bias error, i.e. anoffset component.

An imaginary line extended from the point on support ring 32 at whichforce 44 is applied, through the plane of proof mass 34 (prior to therestoring action of torque coils 36), serves as a reference to define asecond line 46 that is perpendicular thereto and is disposed immediatelyabove the normal position of the centroid of capacitance 42. This secondline 46 is a line about which support ring 32 deflects due to theapplied force 44 and is defined as the "axis of deflection."

Turning now to FIGS. 4 and 5, a first approach is disclosed forcompensating for an applied imbalanced force 44. In this firstembodiment of the present invention, generally denoted by referencenumeral 100, the prior art transducer of FIGS. 2 and 3 is modified bymoving the outermost support pads 30 so that they are closely groupedwith the center support pads 30; thus, all pads 30 are disposed on arelatively shorter segment of the support ring 32 than in the prior artdesign. For the sake of clarity, torque coil 36 is not shown on thisembodiment nor in the figures showing the other embodiments of thepresent invention. As shown in FIG. 5, an axis of deflection 102(defined as was the axis of deflection 46), is aligned with the centroidof capacitance 42. As a result, the imbalanced force 44 does not causethe centroid of capacitance 42 to deflect from its normal position andtorque coil 36 does not produce a restoring force. Thus, the output ofthe transducer is free of any bias signal error due to the deflection ofsupport ring 32.

Turning now to FIGS. 6 and 7, a second embodiment of the presentinvention, generally represented by reference numeral 120, is shownwherein a deflecting force 132 is applied to support ring 32 at a pointwhich is substantially closer to one edge of the cantilever arm than toits other edge, i.e., closer to one of flexures 38 than to the other.Absent any provision for compensating force 132, the prior arttransducer shown in FIGS. 2 and 3 would produce a bias error signal as aresult of the applied imbalanced force 132. To compensate for the "offcentered" deflection of support ring 32 caused by force 132, the secondembodiment of the subject invention 120 is provided a centroid ofcapacitance 128 which is shifted from the previous centroid ofcapacitance 42. The change in the disposition of the centroid ofcapacitance 128 results from provision of an asymmetrical distributionof the pick-off capacitance area. A relatively larger plated capacitancearea 124 is disposed on a portion of proof mass 34 that is relativelyfarther away from the point at which force 132 is applied than is asecond smaller pick-off capacitance area 122. The relative sizes andarrangement of pick-off capacitance areas 124 and 122 are selected sothat the centroid of capacitance 128 is shifted onto an axis ofdeflection 130 associated with the deflection of support ring 32 causedby the applied force 132. Due to the coincidence of the centroid ofcapacitance 128 with the axis of deflection 130, the centroid ofcapacitance 128 is not displaced from its normally centered positionbetween first and second stators 22 and 24 of support ring 32;therefore, an output signal from the transducer to which the secondembodiment 120 is applied does not include a bias error. A mechanicaleffect similar in result can be obtained by making the support ring (onthe side closest to the load) stiffer by increasing its width orthickness or reducing its length, while using pick-off capacitance area40 instead of areas 124 and 122.

A third embodiment of the subject invention is generally denoted byreference numeral 150 as shown in FIGS. 8 and 9. In this embodiment,moment arms 152 are provided on a support ring 154, and are defined by"L" shaped slots 156, which, in a first leg, extend radially outwardfrom the internal circumference of support ring 154, and in a secondleg, are aligned parallel with the circumference. Mounting pads 30 areapplied to the extending ends of moment arms 152, generally lying on aline extending through the centroid of capacitance 42. Centered betweenmoment arms 152 and disposed opposite flexures 38 are other mountingpads 30, as in the first embodiment. As shown in FIG. 9, provision ofmoment arms 152 shifts the axis of deflection 102 so that it is alignedwith the centroid of capacitance 42. Again, a force 44 applied tosupport ring 154 causes it to deflect about axis 102, but does not causeany displacement of the centroid of capacitance 42 from its normalposition. As a result, the output from a transducer incorporating thethird embodiment 150 does not include a bias error due to the appliedimbalanced force 44.

Turning to FIGS. 12 and 13, a fourth embodiment of the present inventionis generally denoted by reference numeral 160. A support ring 162includes an arcuate slot 164 disposed adjacent flexures 38, centered inthe radial extent of the ring, and terminating at each end approximatelyat axis 102. A pair of mounting pads 30 are disposed opposite flexures38, on each side of ring 162. Additional pairs of narrow mounting pads166 are provided on each side of support ring 162, spaced apart frommounting pads 30 approximately one third of the circumference of thesupport ring. A force 44 applied to support ring 162 adjacent flexures38 (i.e., on the portion radially inside slot 164) causes the supportring to deflect downwardly about axis 102, but the centroid ofcapacitance does not deflect from its normal position. Slot 164 thusshifts the bending axis 102 into alignment with the centroid ofcapacitance 42.

Finally, a fifth embodiment is shown in FIGS. 14 and 15 and isidentified by reference numeral 170. As clearly shown in FIG. 14, aproof mass 172 is trimmed to provide a flat side 174 opposite flexures38. A support ring 176 is mounted with support pads 30, in a generallyconventional manner. Pick-off capacitance area 178 is applied to theproof mass in a generally symmetrical pattern relative to the center ofthe torque coil 36, making the centroid of capacitance 42 coincidentwith the center of the torque coil, and with axis 102. A force 44applied to deflect support ring 176 about axis 102 merely causes thetorque coil to pivot about that axis, but does not deflect the centroidof capacitance.

Although it may appear that compensation for an applied imbalanced force44 or 132, by appropriately shifting either the axis of deflection orthe centroid of capacitance into alignment depends upon the magnitude ofthe applied force, it can be shown that this is not the case. Proof ofthe preceding premise is presented herein for a simplistic rectangularshaped support and proof mass assembly, as shown in FIGS. 10 and 11;however, the result applies equally well to a circular or more complexshaped support and proof mass assembly.

Referring now to FIGS. 10 and 11, two parallel supports 200, each ofthickness "h" and width b/2 extend over a length L. Supports 200 arecantilevered from a supporting structure 202 at one end, and at theirother end, are connected along a line from which a proof mass 204 iscantilevered by means of a flexure 206. A central point 208(corresponding to a centroid of capacitance) is selected within theinterior of proof mass 204 at a distance "a" from the outwardlyextending ends of support arms 200, and a force "P" is applied, as shownby arrow 210, to deflect support arms 200 by a distance "δ" as shown at212. The angle of deflection or slope is equal to "θ".

The standard equation for deflection of a rectangular beam having amodulus of elasticity, "E" is given as:

    δ=4PL.sup.3 /Ebh.sup.3 =K.sub.δ P.

The slope at the free end of arms 200 is:

    θ=6PL.sup.2 /Ebh.sup.3 =K.sub.θ P.

However, since a=δ/tanθ, for small angles where tan θ=θ (radians),a=δ/θ. Substituting from the previous equations,

    a=K.sub.δ P/K.sub.θ P=K.sub.δ /K.sub.θ .

Therefore, the position of point 208 described by the length "a" is theratio of two constants which depend on invariant characteristics of agiven support and proof mass assembly and which, for small angles, arenot a function of the magnitude of the displacement. The position wherethe central point 208 crosses the no load position line is independentof the magnitude of the load P that is applied. Thus, it is alwayspossible to align a centroid of capacitance with an axis of deflectionregardless of the magnitude of the imbalanced force applied to thesupport ring (so long as the deflection of the support ring subscribes asmall angle).

It will be apparent to one skilled in the art that the above proof holdstrue for beams of different cross-section and taper. Taking that a stepfurther, it will be apparent that for small displacements of anylinearly elastic structure, the displacement and the local slope arelinear functions of load. Therefore, with an appropriate free proof massstructure, a point of zero relative translation can be found, becausethe distance to this point (similar to "a" in the equation above) isindependent of the applied load. It will further be apparent thatcompensation for an applied force on the support ring 32 or 154 can beachieved by either moving the pads 30 as in the first embodiment 100,moving the centroid of capacitance 128 as in the second and fifthembodiment 120 and 170, or by modifying the support ring 154 as in thethird and fourth embodiment 150 and 160, respectively.

Although the present invention has been disclosed with respect toseveral preferred embodiments, modifications thereto will be apparent tothose skilled in the art. Accordingly, it is not intended that theinvention be limited by the disclosure or by such modifications, butinstead that its scope should be determined entirely by reference to theclaims which follow hereinbelow.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A transducercomprising:(a) a movable proof mass to which is applied a pick-offcapacitance plate having a centroid of capacitance; (b) a support withinwhich the proof mass is mounted, said support including a fixed portionand a cantilevered portion, said proof mass being attached by acompliant cantilever arm to the cantilevered portion of the support andthus movable in a generally transverse direction relative to a planealigned with a surface of the support; (c) a stator assembly in whichthe fixed portion of the support is mounted, the cantilevered portion ofthe support deflecting around a deflection axis with respect to thestator assembly in response to an imbalanced force acting on thecantilevered portion of the support at a point proximate the compliantcantilever arm, where said imbalanced force has a component in thetransverse direction; (d) a plurality of pads disposed between thestator assembly and the fixed portion of the support, said pads beingspaced apart from each other and positioned substantially away from thedeflection axis such that the deflection axis is aligned with thecentroid of capacitance, whereby displacement of the centroid ofcapacitance due to deflection of the cantilevered portion by theimbalanced force is substantially prevented.
 2. The transducer of claim1, wherein the support comprises a generally flat ring and the pluralityof pads are disposed around an arcuate portion of the ring comprisingless than one-third of its total circumference.
 3. The transducer ofclaim 1, further comprising means for detecting a change in the motionof the transducer along the transverse direction by sensing adisplacement of the pick-off capacitance plate, including means forproducing a restoring signal to eliminate the displacement and operativeto produce an output sign that is a function of the restoring signal andthus indicative of the change in the motion.
 4. The transducer of claim1, wherein the plurality of pads are disposed on opposite surfaces ofthe fixed portion of the support and clamp the fixed portion of thesupport within the stator assembly to mount it.
 5. A method forminimizing a bias error in an output signal of a transducer, where thetransducer has a pick-off capacitance plate mounted on a cantilever armextending from a support that is itself mounted as a cantilever, saidmethod comprising the steps of:mounting a fixed portion of the supportto a stator assembly, the remainder of the support comprising acantilevered portion, so that when an imbalanced force acts on thecantilevered portion of the support, said cantilevered portion of thesupport deflects about a deflection axis that is substantially displacedfrom where the fixed portion of the support is mounted to the statorassembly; and controlling the extent and disposition of the fixedportion of the support, so that the deflection axis is aligned with acentroid of capacitance of the pick-off capacitance plate, whereby theimbalanced force is prevented from displacing the centroid ofcapacitance, thereby minimizing the bias error that would otherwise becaused by such displacement.
 6. The method of claim 5, wherein the stepof mounting comprises the step of attaching the fixed portion of thesupport to the stator assembly with a plurality of spaced apart pads. 7.The method of claim 6, wherein the support comprises a generallyflattened ring having opposed surfaces and a circumference.
 8. Themethod of claim 7, wherein the step of controlling the extent anddisposition of the fixed portion of the support comprises the step ofspacing the pads apart about an arc that comprises less than one-thirdof the circumference of the support, opposite ends of the arc beingsubstantially displaced from the deflection axis.
 9. The method of claim8, wherein the pads are mounted between the opposed surfaces of thesupport and the stator assembly, said fixed portion of the support beingcompressed between the pads and the stator assembly.
 10. The method ofclaim 9, wherein the imbalanced force is developed due to a stressapplied to the cantilevered portion of the support that is independentof a force compressing the fixed portion of the support between the padsand the stator assembly.
 11. The method of claim 5, wherein theimbalanced force generally acts on the support at a point that isadjacent the cantilever arm and opposite the fixed portion.